1. Technical Field
The present invention relates to an oscillation circuit and a frequency-correcting oscillation circuit, and more particularly, to a circuit configuration for improving a precision error based on a temperature characteristic having a hysteresis characteristic with respect to a temperature variation.
2. Related Art
The frequency of a piezoelectric oscillator employing a quartz crystal vibrator has a temperature characteristic of a cubic curve in the case of an AT vibrator, but a method of causing a system such as a mobile-terminal GPS system to correct the temperature characteristic for use has been known. For example, JP-A-2003-324318 discloses a piezoelectric oscillator having a function of storing information on a frequency-temperature characteristic of the piezoelectric oscillator in a memory disposed in an IC of the piezoelectric oscillator and outputting temperature information. US Patent Application Publication No. 2006/0071728 discloses that a system corrects a frequency on the basis of temperature information.
Such a TSXO (Temperature Sensing Xtal Oscillator) has a function of outputting temperature detection data and an oscillation frequency at a certain ambient temperature and also has a function of storing the temperature detection data (for example, a temperature sensor voltage and a temperature constant) and the oscillation frequency at a arbitrary temperature in an internal memory. The data stored in the internal memory is data of the frequency measured at plural temperatures while varying the temperature or the temperature information in the process of manufacture and inspection. When an AT-cut quartz crystal vibrator using thickness-shear vibration is used, the frequency of the oscillator draws a positive cubic curve with respect to the temperature.
The system (for example, the mobile-terminal GPS system) derives the relation between the temperature (temperature sensor voltage) and the oscillation frequency using the information in the memory on the basis of the above-mentioned information. In a normal operation, the system detects the temperature output information and the output frequency of the oscillator, predicts a frequency deviation from a reference frequency (for example, a frequency at 25° C.) at the temperature, and determines the frequency correction amount. In this way, the frequency correction is carried out so as to keep the frequency constant at any temperature or the frequency is corrected by software.
As disclosed in JP-A-2003-324318, in a piezoelectric oscillator in which an IC including an oscillation circuit, a temperature sensor, and a memory is connected to a quartz crystal vibrator, it is assumed that the frequency of the piezoelectric oscillator to the original temperature (temperature sensor voltage) is 1:1. For example, FIG. 6 shows the temperature characteristic of a temperature sensor voltage, in which the temperature sensor voltage varies substantially linearly with respect to a variation in temperature. This temperature is a temperature of a semiconductor having a temperature sensor, strictly has a difference from the temperature of the quartz crystal vibrator, and is affected by the structure of the oscillator. FIG. 7 shows the frequency-temperature characteristic of an ideal piezoelectric oscillator. However, in fact, the temperature-rising characteristic and the temperature-falling characteristic differ finely from each other (which is called a hysteresis characteristic).
That is, as shown in FIG. 8, the temperature characteristic of the quartz crystal vibrator may have a hysteresis characteristic in which the temperature-rising characteristic 53 is different from the temperature-falling characteristic 54. In this case, two frequencies exist at a temperature, which causes a precision error. In other words, the temperature sensor voltage and the frequency of the piezoelectric oscillator do not have a complete correspondence of 1:1.
Therefore, in systems such as a mobile-terminal GPS system requiring high precision, it is necessary to improve the hysteresis characteristic. However, to satisfy such a requirement, the structure of the quartz crystal vibrator or the method of mounting the quartz crystal vibrator on a package has to be reviewed and the stable productivity thereof is also required, which cannot be easily handled.